An LVM snapshot is neither a copy nor a reflink: it is a fixed-size copy-on-write exception store attached to an origin logical volume. lvcreate -s -L 5G -n snap vg/data allocates 5 GiB from the volume group, and the device-mapper snapshot target records the original contents of each origin chunk the first time it is written. Creation is instant whatever the origin’s size, because nothing is copied. ...
A ZFS snapshot is a name attached to a dataset’s block tree at one transaction group. Nothing is copied, nothing is quiesced, and the cost is a few metadata blocks: zfs snapshot tank/data@before-upgrade returns before the shell prompt redraws. Because ZFS never overwrites a live block in place, the old tree stays valid on disk until the last reference to it goes away, and the snapshot is that reference. ...
A btrfs snapshot is not a copy of anything. btrfs subvolume snapshot creates a new subvolume whose root points at the same extent tree as the source, so every extent the two share is referenced twice and stored once. Creation is a metadata operation measured in milliseconds whether the subvolume holds a gigabyte or ten terabytes. ...